When Flash first enters the shark men's craft he and Dale are soaked from being in the water. However, in the next shot they are completely dry.
When Flash Gordon and Princess Aura are falling into the pit and land on the net, Aura visibly loses her crown. In the next shot from above we see both lying on the net, obviously unconscious - and Aura's crown is back on her head. In the following shot, a close-up of both waking up again, the crown has disappeared again and does not re-appear in the following scenes.
When the full view of Ming's Throne Room is shown (matte shot), the curtain hanging to the left behind the dragon gong has sunrays on it. When closeups are shown (especially when Flash fights Ming's guards), the curtain has no sunrays on it whatsoever.
When Flash is invisible, when he closes the throne room double doors, there is no way his arms could possibly reach far enough to close both doors at the same time.
When Flash is "underwater" fighting the octopus, his hair and clothes are completely dry.
King Vultan's pet black bear is portrayed as having white markings in the form of stripes. When the king ushers the bear back to its den, he pats it on the back. A small cloud of white dust can be seen. Even though it is apparent, the markings are quite obviously chalked powder.
Before the Hawk Men attack Flash and the others, someone off-screen yells "Get 'em!" The same voice is heard later when, as Flash and Thun rush to stop Ming's marriage to Dale, they into a group of Ming's guards.
When some of the characters speak, the voice on the soundtrack doesn't match the person speaking.
When Ming tells his soldiers to get Aura out of the arena, his lips move but his voice is out of synch. Also, when Aura shoots the guard at the control panel, you hear him scream, but you don't see his lips move.
When Flash (who is invisible at the time) scares the guard in Vultan's cell, you can see the wires that are connected to Vultan's food.
It is never fully explained why Dr Alexis Zarkov has an Irish accent you can cut with a knife.